To understand why the neutrino can provoke a revolution, we need some history. In the early 1960s the situation in theoretical physics was, to put it mildly, chaotic. Particle accelerators were spitting out new particles day in and day out; talk of ‘elementary particles’ was then a laughing matter. But then Murray Gell-Mann came into play, and in 1962 he announced a way of grouping particles that he called “the Eightfold Path”, a clear allusion to Buddhist philosophy. His theory – which was also formulated independently by the Israeli Yuval Ne’eman – launched quarks into the physics arena. Since then, theoretical physicists have been constructing a delicate edifice to describe the world of elementary particles. That edifice is called the Standard Model of particle physics.
It presupposes that there are primarily two families of elementary particles: quarks and leptons. Thus, protons and neutrons consist of three quarks, whereas the electron is a lepton and consists of nothing smaller. It is known that there are six types of quarks and six types of leptons, which are grouped in pairs to form three families, further complicating matters. Briefly, the Standard Model consists of the following particles: up and down quarks, which make up the first family, charm and strangeness, and top and valley.
Leptons, of which there are six, are categorized as follows: the electron and its neutrino, the muon and its neutrino, and the tauon and its neutrino. In a pretentious manner, physicists refer to each of these categories as a flavor, despite the fact that their names have virtually little flavor. The visible stuff around us is composed of the first family of quarks and leptons; the others are utilized to create the strange particles found in cosmic rays and particle accelerators. Small traces of the Standard Model survived at the beginning of this century after its consolidation over the years.
And then came 4 July 2012, the great moment of glory for particle physics. At a crowded press conference at CERN (Geneva), the detection of the particle for which, to a large extent, it was decided to build the LHC accelerator was announced: the Higgs boson. The celebration was justified, because since the detection of the top quark, particle physicists had had nothing interesting to eat for almost 20 years. Its detection gave a definitive boost to the Standard Model, as experiments since 2012 have been confirming that it has the properties predicted by the theory.
Of course, in a dark corner of the model there is an uncomfortable visitor that refuses to adapt to this well-built theoretical edifice: the neutrino. During the second half of the 20th century, as experimentation with this elusive particle progressed, its role in the Standard Model became clearer and clearer: it is the only material particle with zero mass; there are exactly three types of neutrinos – electron, muon and tauon; and neutrinos and their corresponding antiparticles are different, distinguished by a feature called helicity – a version of what in our everyday world is right-handed (dextrorotatory) or left-handed (levorotatory) spin: all types of neutrinos are levorotatory and their antineutrinos are dextrorotatory. This is a permanent property and cannot change (a neutrino cannot be dextrorotatory) because its rest mass is zero.
Everything was going well until the 1960s when Ray Davis of Brookhaven National Laboratory (USA) came up with the idea of studying the neutrinos produced in the Sun by nuclear fusion reactions: every time four hydrogen nuclei are converted into one helium nucleus, two neutrinos are produced, which immediately escape into space. The number of neutrinos available is very high indeed: the Sun produces more than 200 trillion trillion trillion trillion of them every second. But because they are so difficult to detect, Davis had to fill a tank with 600 tonnes of perchloroethylene – a compound used in dry cleaning – and bury it under tonnes of rock; this way it was shielded from any external influences, such as cosmic rays. The surprise came in 1968: Davis detected only a third of the neutrinos predicted by the theory. Thus, was born what has since become known as the “solar neutrino problem”. What on earth was going on in the Sun?
Curiously, the solution had been provided in 1957 by an Italian physicist called Bruno Pontecorvo. Seven years earlier, on 31 August 1950, while on holiday in Italy, he suddenly travelled to Stockholm with his wife and children. The next day KGB agents helped them enter the Soviet Union via Finland, where he was received with honours. In the USSR he received the privileges that this supposedly socialist and egalitarian society reserved for the nomenklatura, the highest-ranking officials.
Pontecorvo showed unparalleled physical intuition, especially in the field of neutrinos. Among other brilliant ideas, Pontecorvo proposed that they could perhaps change suits and become other types of neutrinos, a phenomenon known as neutrino oscillation. Nuclear reactions in the Sun’s interior produce electron neutrinos, so Davis’ team was only detecting that type, but what if on their way to Earth they changed, becoming one of the other two? That was the only possible explanation, but it needed to be demonstrated experimentally.
That came in 1998 from Japanese physicist Takaaki Kajita: he discovered that when cosmic rays hit the Earth’s atmosphere, the muon neutrinos generated ‘change suits’, oscillating, before reaching the detector under Mount Kamioka in Japan. It was the experimental confirmation everyone was waiting for. The neutrinos are going to tell us where future particle physics can move,” commented José Bernabeu, from the Institute of Corpuscular Physics and one of Spain’s leading neutrino experts, who has earned the nickname Don Neutrino for good reason.
Neutrino oscillation is a kick in the stomach of the Standard Model because it requires that neutrinos must have mass (in fact, they do so with a frequency that is proportional to their mass) and the theory says they do not. The physicist Concha González-García, ICREA professor at the Institute for Corpuscular Physics in Valencia and Stony Brook University in New York, one of the world’s leading experts in neutrino theory, has said it clearly: “We are going to find ourselves with a new physics”. What will happen to the Standard Model? Nobody knows, but what is certain is that this almost undetectable particle will tell us where the future physics of the very small may be moving. In fact, it remains to be decided whether the neutrino is of the Dirac type (a particle and its antiparticle are different things) or the Majorana type (the particle is its own antiparticle). What is most fascinating is that no material particle has been found to be of the Majorana type. Will the neutrino be?